1. Field of the Invention
The present invention relates generally to the manufacture of semiconductor devices. More specifically, the present invention relates to improved methods and apparatus for tuning rf matching networks for a plasma processing chamber.
2. Description of the Related Art
Semiconductor processing systems are generally used to process semiconductor wafers for fabrication of integrated circuits. For example, plasmaenhanced semiconductor processes are commonly used in etching, oxidation, chemical vapor deposition, or the like. The plasma-enhanced semiconductor processes are typically carried out by means of plasma processing systems.
FIG. 1 illustrates a representative plasma processing system 100 for processing a semiconductor wafer 102. The plasma processing system 100 includes a plasma processing chamber 104, which is well known in the art. The processing chamber 104 includes an electrostatic chuck 112 for supporting and clamping the wafer 102 in place for plasma processing. The plasma processing system 100 also includes an rf generator 106 and an rf matching network 110 coupled to the rf generator 106 by means of a cable 108. The rf matching network 110 is coupled to deliver rf power from the rf generator 106 to the electrostatic chuck 112.
When the rf generator 106 is energized after a source gas (not shown) has been introduced into the chamber 104, a plasma 114 is created from the source gas. The wafer 102 is disposed over the electrostatic chuck 112 to be processed by the plasma. A heat transfer gas (e.g., helium) 116 may be provided to the wafer 102 under pressure via one or more ports 118 through the electrostatic chuck 112. The heat transfer gas 116 acts as a heat transfer medium between the wafer 102 and electrostatic chuck 112 to facilitate control of the wafer temperature during processing.
In this arrangement however, the rf power supplied to the plasma processing chamber 104 may be reflected back from the plasma processing chamber 104, thereby reducing the efficiency of the plasma processing system 100. The rf power reflection is generally caused by a mismatch in impedance of the rf generator 106 and a load formed by the plasma 114 and the chuck 112. The rf generator 106 has an output impedance Z.sub.0, which is typically 50.OMEGA.. The cable 108 has a matching characteristic impedance equal to the output impedance of the rf generator 106. The plasma 114 and the chuck 112 together form the load characterized by a complex load impedance Z.sub.L. If Z.sub.L is not equal to Z.sub.0 *, which is the complex conjugate of Z.sub.0, then an impedance mismatch exists between the generator and the load.
The rf matching network 110 is provided between the rf generator 106 and the plasma processing chamber 104 to minimize reflection of rf power from the plasma processing chamber 104. The rf matching network 110 typically includes two or more variable impedance elements (e.g., capacitors, inductors). The variable impedance elements may be tuned to provide an impedance Z.sub.M that matches the impedance of the rf generator 106.
FIG. 2 shows a circuit diagram of an exemplary rf matching network 110 coupled to the load 202, which is equivalent to the combination of the electrostatic chuck 112 and plasma 114. The rf matching network 110 includes a variable capacitor C1 coupled to an inductor L1 in series. The rf matching network 110 also includes a variable capacitor C2 coupled in series to an inductor L2. The variable capacitors C1 and C2 are coupled to each other at a junction A. The electrode and plasma load 202 is coupled in series with the inductor L2 and is coupled to a junction B.
In this configuration, the variable capacitors C1 and C2 may be tuned to provide an impedance Z.sub.M across the junctions A and B, which matches the impedance of the rf generator 106. The impedance, Z.sub.M, represents the total impedance of the network 110 in combination with the load 202. Ideally, when the impedance Z.sub.M is equal to the output impedance of the rf generator 106, the rf power reflected is at zero percent.
For example, if the impedance of the rf generator 106 is 50.OMEGA., then the magnitude and phase of complex impedance Z.sub.M need to be equal to 50.OMEGA. and zero degrees, respectively, in order to minimize power reflection. The set of values of the capacitors C1 and C2 at which the complex impedance Z.sub.M equals the output impedance of the rf generator 106 is referred to as a "tune" point or target point. Accordingly, the tune or target point is where the power reflection is at a minimum.
Several techniques are known for tuning variable impedance elements in an rf matching network. FIG. 3 illustrates a flow chart of a conventional method for tuning capacitors C1 and C2. The method starts in operation 302 and proceeds to operation 304, where the plasma processing system 100 including the rf generator 106 is activated. At this time, the capacitors C1 and C2 are usually not set properly to the tune point. Thus, some rf power is reflected back.
Then in operation 306, the magnitude and phase of Z.sub.M are determined by measuring a voltage V, a current I, and an angle .theta. between the voltage and current in accordance with well known equation Z.sub.M =.vertline.Z.sub.M.vertline.e.sup.i.theta., with .vertline.Z.sub.M.vertline.=.vertline.V.vertline./.vertline.I.vertline.. In operation 308, it is determined whether the magnitude of Z.sub.M is equal to a tune point value, for example, of 50.OMEGA.. If not, the operation proceeds to operation 310, where the variable capacitor C2 is adjusted by means of a computer and DC motors to match the impedance of the rf generator 106. If the magnitude of impedance Z.sub.M is greater than the impedance of the rf generator 106, then the capacitance of C2 is increased. Conversely, if the magnitude of impedance of Z.sub.M is less than the impedance of the rf generator 106, then the capacitance of C2 is decreased.
After adjusting capacitor C2 in operation 310 or if magnitude is equal to the tune point in operation 308, the method proceeds to operation 312, where it is determined if the phase is equal to zero degrees. If the phase is determined to be non-zero, the method proceeds to operation 314, where capacitor C1 is adjusted to change the phase to reach the target impedance phase of zero degrees. For example, if the phase of the impedance Z.sub.M is less than zero, then the capacitance of C1 is increased. Conversely, if the phase is greater than zero, the capacitance of C1 is decreased. It should be noted that the variable capacitors C1 and C2 are adjusted by means of a computer and DC motors. Specifically, a computer may drive the DC motors to adjust the capacitance of the capacitors C1 and C2 so as to reach the target tune point.
After adjusting capacitor C1 in operation 314 or if the phase is equal to zero in operation 312, the method proceeds to operation 316, where it is determined whether the plasma processing is complete. If so, the method terminates operation 318. Otherwise, the method proceeds back to operation 306 to continue tuning the capacitors C1 and C2 in the continually varying conditions (e.g., varying load, drifting tuning motors) of the plasma processing system.
Unfortunately, the method described in FIG. 3 may not efficiently tune the capacitors to the target point for certain ranges of capacitance values. The problem is illustrated more clearly in FIGS. 4A and 4B. FIG. 4A illustrates an exemplary graph 400 plotting the magnitude 402 of impedance Z.sub.M and reflected power 404 as a function of the value of capacitor C2 for capacitor C1 held fixed at its tune value. The tune value of capacitor C2 is about 231 pF.
A line 406 indicates the impedance tune value of 50.OMEGA.. The magnitude 402 and the tune line 406 intersect at points 408 and 410. The point 410 represents the tune point at which the power reflected is at a minimum while the point 408 corresponds to a capacitance value of about 255 pF. At capacitance C2 values of less than 255 pF, the tuning method of FIG. 3 works efficiently by increasing C2 if .vertline.Z.sub.M.vertline. is greater than 50.OMEGA. and decreasing C2 if .vertline.Z.sub.M.vertline. is less than 50.OMEGA.. However for C2 values above 255 pF of point 408, the above method adjusts the value of C2 in the wrong direction. For instance at C2 values above 255 pF, the method increases C2 even though C2 is already above its tune value, thereby moving away from the tune point.
Similarly, the method of FIG. 3 does not efficiently tune the impedance angle .theta. in some range of C1 values. FIG. 4B shows a graph 450 plotting the phase .theta. 452 of the impedance Z.sub.M and reflected power 454 as a function of the capacitor C1. In the graph 450, the capacitor C2 is held fixed at its tune value of 231 pF. A tune line 456 represents the .theta. value of zero degrees. The phase line 452 and the tune line 456 intersect at points 458 and 460. The point 460 represents a tune point where the power reflected is at a minimum. The value of C1 at the point 460 is about 837 pF,
The point 458 corresponds to a capacitance value of about 800 pF. At C1 values of greater than the capacitance at the point 458, the method of FIG. 3 will increase C1 if .theta. is smaller than 0 degrees and decrease C1 if .theta. is larger than 0 degrees. However, for C1 values below the capacitance at the point 458, the method decreases C1 even though C1 is already below its tune value. Hence, the method adjusts C1 in the wrong direction. Thus, by adjusting C1 and C2 values into wrong directions for certain capacitance value ranges, the method may never find tune point or may substantially delay the determination of the tune point.
Another method for controlling a matching network is described in U.S. Pat. No. 5,689,215 by Richardson et al., which is incorporated herein by reference. This method physically changes the values of C1 and C2 in a prescribed order and measures the percentage of reflected power as a function of capacitor value as each capacitor is varied. In so doing, the method finds two pairs of values (C1, C2) that correspond to local minima in the reflected power. A straight line connecting these two pairs of values points generally toward the true tune point. The method varies the values of C1 and C2 to follow the straight line. This process is then repeated until the desired level of reflected power is obtained.
While the method described in U.S. Pat. No. 5,689,215 works well, it may require substantial time to find a tune point. This is because physically varying the capacitors generally requires more time than computer implemented calculations.
U.S. Pat. No. 5,793,162 by Barnes et al., which is incorporate herein by reference, also describes a technique for controlling matching network of a vacuum plasma processor. The technique for controlling a matching network also relies on measurements of reflected power. Specifically, the technique involves varying a capacitor in one direction. If the reflected power increases, the adjustment direction of the capacitor is reversed. On the other hand, if the reflected power decreases, the adjustment direction of the capacitor remains the same as long as the reflected power continues to decrease. While this technique works well, it may not produce tune points in a speedy manner. For example, the reversals of directions may further add to the time required to find the tune point.
Yet another method for tuning a matching network using a predictor-corrector control system is described in U.S. Pat. No. 5,187,454 by Collins et al., which is incorporated herein by reference. This method involves estimating the values of matching network values at their tuned condition and moving the network values toward the estimated tune position. Unfortunately, this method also adjusts the variable impedance elements of the match network in the wrong directions under some circumstances. This can occur, for example, when the load impedance is not constant but is itself a rapidly varying function of the values of the variable impedance elements.
In view of the foregoing, what is needed are methods and systems for more rapidly and stably tuning an rf matching network to deliver maximum power to a vacuum plasma processing chamber.